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I have a ceramic mortar & pestle, approximately 10 cm across, that I use primarily for grinding whole spices. When I bought it about 15 years ago, only the exterior of the mortar (and a small amount of the interior) was glazed; the interior had an unfinished, rough texture akin to bisqueware. Similarly, the bottom 2–3 cm of the pestle was also rougher in texture.

After many years of usage, this rougher texture has been smoothed out in the center of the mortar and at the end of the pestle. I think (though it may just be confirmation bias) that this makes it less effective at grinding, as the spices are not "gripped" as much by the ceramic surfaces.

Is there a good way to re-roughen the grinding surfaces of this mortar & pestle? Or would it not be worth the trouble to do so?

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Potter and maker of ceramic mortars here.

Should You: whether or not a mortar actually becomes smooth enough to affect grinding inside depends a lot on what kind of ceramic material the mortar is made from, and what temperature it was fired to. I have a steel-furnace-fired kaolin-based pharmacy mortar that's over 100 years old that I still use, and it's just rough enough (sandstone-like) for good grinding. On the other hand, many inexpensive handmade ceramic mortars are made from low-fire earthenware, and will totally grind smooth with use. Once the interior reaches the smoothness of 'polished wood', it's going to be harder to grind seeds (but see below). Mashing of leaves, roots, and aromatics should be unaffected.

How To: sandpaper. Get some 20 to 50 grit (depending on how rough you want it to be) wet/dry sandpaper. Dampen the mortar (you do not want to breathe the dust) and sand away. The carborundum of the sandpaper is harder than any but the hardest ceramics, and it should roughen it nicely. Then scrub out the mortar and let dry. If you have trouble sourcing low-grit wet/dry sandpaper, use a very rough grinding stone with a Dremel-like tool instead.

Think About: if what you do is primarily grind spices that come in the form of seeds, think about getting a Japanese-style "combed" texture mortar (see pic). These mortars are specifically designed for grinding seeds, and the rough interior won't grind smooth, particularly if you use the recommended hardwood pestle.

japanese-style combed texture mortar

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    Dang. Now I have a new kitchen gadget for my wish list. <totters off to see whether I can find space>
    – Stephie
    Dec 2 '21 at 18:58
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    "you do not want to breathe the dust" - I wonder about the advisability of ingesting the "ground-smooth" dust over the years (genuine ignorance here, not an allusion). If this may be problematic, maybe replacement is advisable in any case (with something more wear resistant)? But my instinc says it may not be significant...
    – frIT
    Dec 3 '21 at 9:29
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    I have a boron carbide mortar that would argue with your claim regarding carborundum, but I guess that would be covered by the "but the hardest ceramics" part. +1 Anyway. Funfact, you can generally roughen a surface with a softer material it just takes more time. Carborundum sandpaper should do fine. Dec 3 '21 at 9:38
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    @frIT ingestion of (most) mineral dusts is far less of an issue than inhalation - we have far better mechanisms for removing bits of unwanted but not actually toxic stuff from our guts than from our lungs
    – Chris H
    Dec 3 '21 at 15:04
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    @Fuzzychef It was originally meant to grind and mix ceramics before shaping and sintering but the project got canceled and the lab shut down for renovation. Since it was still newly purchased, still in its wrapping I decided to adopt it for culinary purposes. It does fine for that but the metallic sound it makes when you pound seeds is unnerving. It is like striking an ambolt with a hammer, only an octave or two higher in pitch. Dec 4 '21 at 8:38

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